libertas: Consolidate lbs_host_to_card_done() function.

As we move towards having this done by a state machine, start by having
a single 'stuff sent' function, which is called by if_usb/if_sdio/if_cs
after sending both data and commands.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>